For years, electric fences have been used in agriculture settings for the purpose of containing livestock and/or repelling predators. Generally, the electric fences involve non-insulated fence wire being suspended in the air through the use of insulated members (e.g., made of plastic, porcelain, etc.) and being energized by an electric fence controller or energizer. The electric fence energizer is generally provided with two output terminals. In one typical configuration, one of the terminals is connected to the electric fence wire while the other terminal is connected to Earth ground. In turn, any animal that comes in contact with the fence wire while simultaneously being in contact with the ground receives an electric shock. Alternatively, the electric fence energizer's output terminals may be connected to two fence wires positioned one above the other. In turn, any animal coming in contact with both fence wires simultaneously receives an electric shock.
As is known, the majority of present electric fence energizers employ capacitive discharge systems. The primary elements of such systems include (i) a capacitor (or bank of capacitors), (ii) a step-up transformer, and (iii) a switch such as a silicon controlled rectifier (SCR). During normal operation of these systems, the capacitor is charged to a DC voltage over a period of time (e.g., typically about one second), and upon activation of the switch, is discharged rapidly through a primary winding of the transformer. The result is a high voltage pulse (e.g., typically around 10,000 volts DC) on the transformer's secondary winding, which is, in turn, applied to the fence wire for the purpose of deterring animals.
During a lightning strike, voltage can be induced onto the fence wire and back to the secondary winding of the step-up transformer. As a result of the induced voltage from the lightning, the transformer is subjected to very high voltages. While fence energizer transformers in the past may have become routinely damaged from such voltage levels, fence energizer are now commonly equipped with transformers that can withstand tens of thousands of volts. Consequently, fence energizer transformers are commonly found capable of withstanding voltages in excess of 25,000 volts.
As described above, the transformer in an electric fence energizer is configured to step up the voltage from a capacitor during normal operation; however, the reverse functioning also applies for the transformer. That is, when lightning induces a voltage on the fence wire and secondary winding of the step-up transformer, the transformer functions to step down the induced voltage on to the primary winding of the transformer. Unfortunately, while transformers designed to withstand the induced voltages from lightning strikes are commonly used in fence energizers, the transformers offer little to no protection to the circuitry on the primary side of the transformer, as described below.
In low power electric fence energizer designs, the transformer's primary to secondary turns ratio is generally high (e.g., in many typical designs, the turns ratio is around 1:50). Consequently, voltage induced on the primary winding from a lightning strike would be relatively low (e.g., using an exemplary turns ratio of 1:50, a 25,0000 volt surge induced on the fence wire/secondary winding would result in about 500 volts on the primary winding). Thus, in such a low power circuit, the energizer switch on the primary side of the transformer would only need to withstand induced voltages ranging in hundreds of volts given a 25 KV surge from the fence wire. Because switches designed to withstand several hundred volts are commercially available (e.g., an 800 volt SCR is common), no additional lightning protection circuit is typically needed with this type of low power energizer. However, in higher power fence energizer designs, the transformer's primary to secondary turns ratio is generally much lower (e.g., in many typical designs, the turns ration is around 1:10). Thus, given the same 25,000 volt surge on the fence wire and transformer secondary winding, the primary winding will have an induced voltage of 2,500 volts. Switches (such as SCRs) can be typically found to withstand voltages as high as 1000 volts; however, switches with higher voltage ratings are quite expensive.
Given the above, there have been a wide variety of circuit designs used in high power fence energizers to address the induced voltages coming back through the transformer from lightning strikes. In particular, these designs have generally involved using electronic protection devices within the circuitry on the transformer's primary side to limit the amount of induced voltage that the energizer switch sees. Unfortunately, each of these circuitry designs has been found to have shortcomings, as described below.
For example, one method of lightning protection for the primary-side circuitry has involved electrically connecting several switches, e.g., SCRs, in series to withstand higher voltages being induced from lightning strikes. Unfortunately, the use of series-connected switches generally requires a trigger circuit to fire all the switches at once during normal operation and would add considerable cost to the product. Other methods of lightning protection have involved electrically connecting electronic protection devices across the transformer's primary winding and/or the energizer switch, with such devices including Metal Oxide Varistors (MOVs), Transient Voltage Suppressors (TVSs), and diodes. However, these methods have also been found to have drawbacks, as described below.
Use of MOVs is generally limited to smaller devices as larger devices (and their associated higher junction capacitance) are found to cause small current spikes in the energizer switch, which can damage the switch during normal operation. Further, MOVs wear out with use. In turn, as surges are diverted during the operation of the MOV, the life span of the MOV shortens, and failure becomes imminent. TVS devices are similar to MOVs in function. As such, TVS devices again offer limited protection, yet with considerable cost. In comparison to MOVs or TVS devices, a diode placed across the SCR or transformer's primary winding represents the lowest cost option, while providing some protection. Unfortunately, while the MOV and TVS devices remain off until the voltage on the transformer's primary winding is high enough to cause the devices to turn on, the diode turns on immediately. This provides good protection for the energizer switch initially, but is also found to create extremely high currents in the diode (e.g., quite often, several hundred amperes), thereby causing the diode to quickly fail due to overcurrent.
Therefore, it would be advantageous to provide a lightning protection circuit for an electric fence energizer that addresses one or more of the above limitations.